Video amplifier with separate channels for high and low frequencies



Dec. 19, 1950 o. H. SCHADE 2,534,627

VIDEO AMPLIFIER WITH SEPARATE 'CHANNELS FOR HIGH AND LOW FREQUENCIES Filed May 22, 1946 mme Q s. QS as mm2 umn SR# AAA...

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aNvENToR 0770 H. .SCHADE ATTORN EY A. An v v Patented Dec.. i9, 195() VIDEO AMPLIFER WITH SEPARATE CHAN NELS FR HIGH AND LOW FREQUENCIES @tto H. Schade, West aldweil, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application May 22, 1946, Serial No. 671,586

9 Claims.

The present invention relates to an improvement in video amplifiers to be used in connection with a television pick-up or camera tube of the Orthicon type wherein an electron multiplier is provided in the tube and wherein the video sign-als are derived both from the signal plate or target electrode of the Orthicon tube and from the final collector electrode of the electron multipiier within the tube.

In a television pick-up tube of the Orthicon type, a low Velocity cathode ray beam is provided for scanning a target surface upon which a positive electrostatic charge image is produced. Only Such electrons as are necessary to substantially cancel the positive electrostatic charge image at the target surface, or to bring the surface to a uniform datum potential level, are removed from the scanning cathode ray beam and the remaining electrons constituting the beam are returned through the tube along substantially the same path over which they travelled in reaching the Vicinity of the target electrode. In certain types of Orthioon tubes, however, lifter plates have been provided in order that different paths may be traversed by the outgoing or scanning beam and the returned beam cf electrons. Regardless of the actual path traversed by the returning beam of electrons, this returning beam is current modulated-due to the subtraction of electrons from the scanning beam by the target surface to at least partially cancel the electrostatic charge image thereat, and the current modulation of the returning beam of electrons represents a source of video or image signals.

It is frequently desirable to incorporate within the Orthicon, one or more stages of electron multiplication in order to increase the video signal intensity so that the signal-to-noise ratio of the signal obtained from the Orthicon tube may be materially improved.

In Orthicon tubes where the returning cathode ray beam traverses substantially the same path as the outgoing or scanning cathode ray beam, the first secondary electron emitter of the electron multiplier (i. e., the iirst stage of the electron multiplier) is generally positioned relatively adjacent the electron gun structure and. in some instances, is a part of the beam forming electrode of the electron gun structure. This beam forming electrode is provided with a very small aperture (of the order of a few mils in diameter) in order tc reduce the spot size of the scanning cathode ray beam. In spite of this location of the rst stage of the electron multiplier, the rei turning beam of electrons does not continuously impinge upon the identical area of the surface of this emitter but, instead, moves about to a certain extent in accordance with the horizontal and vertical scanning deflections of the scanning beam. The returning beam of electrons, therefore, actually scans a Very small and reduced portion of the surface of the rst stage of the electron multiplier even though this surface is located very closely adjacent to, and frequently forms a portion of the beam forming aperture of the electro-n gun structure. This small move.- ment or scanning action of the return beam over the surface of the first stage of the electron multiplier produces undesired variations in the current intensity of the electrons derived from this emitter of the multiplier other than those current variations brought about' by reason of the current modulations of the returning cathode ray beam. This undesired variation is by reason of the fact that different portions of the surface of the secondary emitter are not absolutely uniform or identical in emission response and as the rcturning cathode ray beam moves slightly over the surface of this first stage of the multiplier, undesired variations occur in the amount of current obtained from this emitter.

If the surface of the rst stage of the electron multiplier were absolutely uniform in its secondary emission response, then the current Variations of the electrons produced by this emitter would bc proportional to the current modulations of the returning cathode ray beam. Under such conditions, no undesired modulations would exist and no undesired dark spot signal would be present. When an undesired modulation in the current obtained from the first stage of the multipler (as a result of the non-uniform emission characteristics Vover the surface of the emitter) is superimposed upon the video signals, this undesired current modulation produces a dark spot in the resultant image. In previous circuit arrangements, this undesired dark spot has been substantially eliminated by dark spot compensation injection as is conventional practice where pick-up tubes of the Iconoscope type are employed. Such a method of reducing the effects of the dark. spot signal is entirely satisfactory, however, rather than reduce the undesired dark spot to4 a negligible value by shading signal injection, it is preferable to effectively eliminate the productitn of the darli spot signal and thus preclude its presence in the video signals thereby obviating the necessity of compensating for dark spot by shading control signal injection.

The United States Patent 2,525,105, issued to Paul K. Weimer on Ocwber l0, 1950, discloses an arrangement of and method for operating a television camera or pick-up tube of the Orthicon type wherein it is possible to produce video signals in which undesired dark spot signals are not present even though an electron multiplier is used in the tube. As explained in the above referred to l/Veinier patent, the undesired dark spot signals are eliminated from the video signal by obtaining from the electron multiplier only such video signals as fall in a. frequency spectrum in which no undesired duri; spot signals are inherently present. Since the dark spot signals are represented by relatively low frequencies, if the Video signals that are derived from the final collector electrode of the electron multiplier are confined to that portion of the frequency spectrum above, for example, 200 kilocycles per second, then no darla spot signal will be present since the signals representing the darli spot are substantially entirely confined to freuencies lower than 200 kilocyc es per second. In order to supply those portions of the vdeo signal spectrum below, for example, 260 kilocycles per second, the above mentioned W -mer application proposes deriving such a portion of the video signal from the signal plate of the target electrode of the Orthicon tube. No dark spot signals are present at this electrode since in the Orthicon, no secondary electrons are produced by the scanning action of the scanning cathode ray beam. The low frequency components of the video signals are, therefore, obtained from the signal plate of the target electrode whereas the nigh-frequency components of the video signals are obtained from the final collector electrode of the electron multiplier in the Orchicon pick-up tube. Such an arrangement does not defeat the purpose of the electron multiplier since the decidedlf,7 improved signal-to-noise ratio may still be maintained in the high-frequency components where an improvement in the signalto-noise ratio is most desirable.

The low and high frequency components so obtained from the television camera or pick-up tube are then combined and in combining the signals it is necessary that they be combined in proper` relative amplitudes and that the phase relationship of the combined signals be proper in order that there will be present no phase distortion or phase shifting throughout the entire video spectrum. After the signals have been so ycombined, they are then applied to further video amplifiers and the video signal is then used to modulate a radio frequency carrier after the necessary blanking, synchronizing and other pulses have been added in a manner well known to those skilled in the art.

The present invention is, therefore, concerned primarily with the combining of the low and high frequency components of the video spectrum in order that the reiative amplitudes of the signals in these two portions as well as their phase relationship will be proper so that no distortion or undesired eiiects will be produced. By reason of the presence of the electron multiplier, the high frequency lcomponents of the video signals (i. e. those over, for example, 200 kilocycles per second) will have an intensity as derived from the tube considerably in excess of the intensity of the low frequency components of the video signals as derived from the signal plate of the target electrode. The low frequency components will, therefore,

require amplification before the signals can be combined.

As a matter of fact, a complete video signal series is obtainable from both the signa-l plate and from the collector electrode of the electron multiplier. No undesired shading signals are present in the video signals from the signal plate. However, as stated above, undesired dark spot signals are frequently present in the video signal series as derived from the electron multiplier. In the present invention signals from each of these two sources are applied to two separate amplifiers which have different frequency characteristics and which operate to amplify and attenuate different portions of the video signal spectrum. For example, the amplifier to which the video signals derived from the signal plate of the target electrode are applied is in effect a low pass amplifier, in which signals below approximately 200 kilocycles per second are amplicd and passed through the amplifier whereas signals above 200 kilocycles per second are attenuated. The amplier to `which are applied the video signals as derived from the electron multiplier is in effect a high-pass amplifier where signals up to approximately 260 kilocycles per second are attenuated 'whereas signals above 200 kilocycles per second are amplified and are permitted to pass through the amplifier. The overall gain of each of the two amplifiers is so adjusted with respect to each other that the output of each amplifier (i. e those portions of the video signal spectrum that are passed by each amplier) bear a proper relative signal intensity so that they may be satisfactorily combined. Some provision is naturally made whereby the relative amplitudes of the signais at the outputs of the ampliers may be regulated and controlled.

In addition to the requirement that the relative amplitudes of the video signal output from the ampliers be properly related. it is also nec-- essaiv that each amplifier exhibit inverse ch"r acteristics insofar as phase distortion of the j nals is concerned so that if any undesired phase displacement occurs in one of the amplifiersI it is compensated by an equal and opposite displacement in the other ainpliiier (in th: frequency region of overlap of the two amplifiers). Furthermore, the phase s iitiug i' e amplifiers must be so compensated that, whe the two portions of the entire video spectrum from the two ampliiiers are combined, any phase shift that may be present in the combined video signals will be directly proportional to frequency so that no distortion will be introduced. After the signals are combined, theY entire spectrum of the video signa-ls is again present in a single channel and these signals may be applied to further amplier and, ultimately, the signals may be used to amplitude or frequency modulate a radio frequency carrier after the necessary blanlring and synchronizing impulses and other necessary ccntrols have been exercised on the video signal series.

It is, therefore, one purpose of the present invention to provide, in a television transmitter where a television camera pick-up tube of the Orthicon type is employed and in which the pickup tube includes an electron multiplier, a pair of pass-band ampliiiers to each ci which is applied individually, Video signals from the signal plate of the pick-up tube and video signals from the electron multiplier or" the pica-up tube.

' Another purpose of the present invention resides in the use of separate high-pass and low- 5, pass video signal amplifiers for amplifying video signals obtained from two separate videol output conductors associated with an Orthcon tube in which the ampli s provide video signals of proper relative amplitudes and of proper phase relationship so that they be combined.

Still another purpose of the present invention resides in applying and related television Video signals to two sep "ate ampi 'tiers in which one amplifier operates increase the intensity of one portion of the video spectrumI while the other amplier operates to increase the intensity of .the other portion of the video spectrum together with circuit arrangements whereby the relative amplitudes .and phase relationship of the signals available from the outputs of the two amplifiers are. suon that the amplined signals may be combined to a normal signal.

A still further purpose of the present invention resides in the provision two amplifiers each of which are capable of amplifying dierent portions of the spectrum of a video signal series and in which the degree of undesired phase displacement present in the amplify` s is substantially compensated that signals ava lable at the outputs of the amplifiers may be combined without introducing phase distortion.

Various other purposes and present invention will become more apparent to those skilled in the ar from the following detailed description particularly when considered in connection with the drawing, wherein:

Figure l. represents a preferred form of the present invention; and

Fig-ure 2 represents curves used in explaining the theory of operation of the present invention.

Referring now to the drawings, and, more particularly, to Figure l thereof, there is shown an Orthicon camera or image pick-up tube il) of the image intensifier type. rEhe tube includes an electron gun structure having a cathode i2, a ccntrol electrode and an accelerating electrode it. This electron gun structure is positioned at one end of the tube and functions to generate a narrow focussed cathode ray beam. The accelerating electrode lil is provided at its forward end with a at surface ES which is in fact the first emitter of an electron multiplier as will be described later. The end surface i8 is provided with a small a erture (of the order of 2 or 3 mils in diameter) i ch functions as the beam forming aperture oi the electron gun structure.

The opposite end of the Orthicon tube is provided with an electrically conducting light responsive surface 'E upon which an optical light image of an object area 2li is projected by means of a lens system Behind the light responsive surface of the photo-cathode r2 is a target electrode 2S generally made of a Wafer of very thin glass. Adjacent one face of the target electrode 2S is a fine mesh screen 29 which forms a signal plate. rThis conducting screen or signal plate is in electrcal contact with the photo-electron accelerating electrode element The accelerating electrode 3&3 and the conducting screen or signal plate are nonmally operated at a substantially fixed potential generally at or near ground potential.. rihe conducting photo-cathode 22 is operated at a negative potential with respect to the signal plate and intermediate the photo-cathode 2212 and the signal plate (or target electrode 2S) is an electrode which is maintained at an intermediate potential. he electrode 32 is, therefore, positive with respect to the photo-cathode yet negative with respect to the signal plate. When an advantages of the optical image is projected upon the photo-cathode 22, photo-electrons are emitted from the surface of the photo-cathode. in an amount depending upon the intensity of the optical image. rllhese photo-electrons are then accelerated in orderl that they impinge upon the front surface of the target electrode with sufficient velocity to generate secondary electrons, which in turn are absorbed by the screen 2li associated with electrode te. By reason of the emission of photoelectrons an electronic current image is present between the. photo-cathode and the target electrode and since secondary electrons of a ratio greater than unity are produced at the target electrode due to the impacting photo-electrons, an intensified. positive electrostatic charge image is produced over the surface of the target electrode corresponding to the optical image projected on the photo-cathode.

Intermediate the target electrode 28 and the.` electron gun structure are additional electrodes 34, t@ and 38 to which are app ied potentials in an increasing positive polarity. 'ihe potentials of these electrodes assist in maintaining proper velocity of the scanning cathode ray beam at the target electrode as well to accelerate and direct the return beam of electrons to the rst emitting surface i8 of the electron multiplier. The path of the outgoing scanning cathode ray beam 6l) and the return cathode ray beam 62 is represented by broken lines in Figure l of the drawmg.

Surrounding the image Orthicon tube ll is a focussing coil 4U to which is applied a direct current for producing an electromagnetic field eX- tending through and parallel to the axis ofsymmetry of the tube. This electromagnetic field assists in maintaining a desired focal condition of the scanning and return cathode ray beams. Positioned between the focussing coil' llc and the pick-up tube it! are the deflection coils 42. These coils surround only a portion ofthe lengthk of the Orthicon tube and to these coils are applied appropriate horizontal and Vertical deflection voltage variations from appropriate deflection generators (not shown) rlhe deflection coils produce alternating electromagnetic fields` in both horizontal and Vertical directions at different rates in order to affect the desired scanning operation as is well known to those skilled in the art. The resultant field produced by the combination of the eld produced by the deflecting coils and the field produced by the focussing coil bring about the desired deflection in the cathode ray beam, yet the desired focal condition of the beam is maintained during its travel to and from the target surface. Appropriate relative potentials may be applied to the various electrodes included within the Orthicon tube il? by means of a potential divider or by any other appropriate means.

The electron multiplier includes as its first emitting surface the end i8 of the accelerator I6 of the electron gun structure and the secondary electrons emitted at this surface are directed` upon the second stage it of the electron multiplier. The electrode 38 assists in persuading or directing these secondary electrons upon the second stage at of the multiplier and this; stageof' the multiplier is annular in constructionl and surrounds the electron gun structure. Naturally, the. emitting surface of this stage of the multiplier is maintained ata potential more positive than the. rst emitter I3. The secondary electrons available from the second stage 4t of the multiplier;

are directed against the third stage 48 where additional secondary electrons are produced and these in turn are directed against the fourth stage 50 of the multiplier where the produced secondary electrons are directed against a collector electrode 52. In the multiplier shown in the drawing, four stages, each including secondary electron emitters having a ratio greater than unity, are provided and these stages are represented at I8, 45, 48 and 55. It, is, of course, obvious that a greater or lesser number of stages may be included in the electron multiplier. The collector electrode 52 is, of course, maintained at the most positive potential and between the collector electrode 52 and the positive terminal 54 oi a source of potential is a load resistance 55 across which video signals may be obtained. Conductor 58 is provided for obtaining the video signal series from the electron multiplier portion of the Orthicon tube.

When the cathode ray beam is directed toward the target electrode 28 and when a positive electrostatic charge image is present at that electrode, a certain number ci electrons constituting the beam will be utilized in neutralizing at least a portion of the electrostatic charge image while the remaining electrons are returned and directed upon the first emitter i8 of the electron multiplier. By reason of this removal of a continually varying number of electrons from the scanned beam 6G, a current modulation is present in the return beam 62 and this current modulation is representative of the light values of the optical image provided on the photo-cathode 22. Due to the scanning action, video signals are also available from conductor which is connected to the conducting surface or signal plate on the front side of the target electrode 28.

In television pick-up tubes of the Orthicon type Where the video signals are derived solely from the returning cathode ray beam, with or without an electron multiplier, and where no video signals are derived from the signal -plate of the target electrode, blanking during beam return time is normally accomplished by applying a voltage variation of rectangular waveform to the target electrode. This voltage variation is represented as negative pulses which occur in phase with the horizontal and vertical return deflections of the scanning cathode ray beam, and, inasmuch as they extend in a negative direction, they preclude the absorption of any of the electrons constituting the scanning cathode ray beam during both the horizontal and the vertical return strokes. A correct black level is, therefore, established in the signal series. In the subject invention, however, such a form of blanking cannot well be employed since video signals are derived from the signal plate associated with the target electrode. If the above referred to type of blanking were employed, a rather objectionable voltage variation would be introduced into the amplifier associated with the signal plate of the target electrode. Since it is preferable not to inject such a, signal into that ampliery blanking in the present invention is accomplished by modulating both the cathode and control electrodes of the electron gun structure. In order that a modulation may be simultaneously applied to these two electrodes of the gun structure, the cathode l2 of the electron gun is connected to ground (or a point of xed potential) by means of a resistance or impedance 63 and a further resistance or impedance S8 is included between the control electrode and the source of biasing potential. A condenser 10 is connected between the control electrode I4 and the cathode l2. Blanking potentials of a waveform such as represented at 12 are then applied between the cathode l2 and ground (or the point of fixed potential) and by reason of the presence of the condenser 'i0 these same blanking impulses are applied to the control electrode I4. The presence of resistances or impedances 66 and 63 permit the potentials of these two electrodes to be modulated. The blanking impulse is in a positive direction and as a result the velocity of the generated cathode ray beam is reduced during both the horizontal and vertical return deflections. Accordingly, by so reducing the velocity of the beam it is preeluded from coming suiciently close to the target electrode to permit any electrons to be absorbed therefrom during the fly-back time. It is desirable that both the cathode and the control electrode be simultaneously modulated by substantially the same extent in order that the current intensity of the cathode ray beam will not be altered during the blanking interval since it is imperative that the current intensity of the developed cathode ray beam be maintained substantially constant. If the current intensity of the cathode ray beam were altered during the blanking interval, then a Corresponding disturbance on signals would be generated in the video signal series as derived from the electron multiplier by way of conductor 58 since the current modulation of the generated cathode ray beam would be reected into the amplifier by the electron multiplier. Accordingly, by modulating both the cathode and the control electrode of the electron gun structure in a positive direction during both the horizontal and vertical return strokes of the cathode ray beam, an effective blanking impulse can be applied to the Orthicon and a true black level signal will be obtained from both conductors 58 and 64 during the blanking interval. Technically, the extent to which the cathode and control electrode are pulsed in a positive direction during blanking should not be identical since a change in the cathode potential in a positive direction effectively amounts to a change in the screen grid potential in a negative direction. Accordingly, the extent to which the control electrode is pulsed in a positive direction might, in some instances, be preferably less than that of the cathode.

As explained above, the returning cathode ray beam 62 does not continuously impinge upon the same area of the rst stage I8 of the electron multiplier but instead actually scans a small area or portion of this electrode in accordance with the deiiection forces applied to the scanning cathode ray beam. Since the surface i8 is not entirely uniform insofar as its secondary electron emission ratio is concerned, an undesired variation in the number of secondary electrons produced at the rst emitter I8 takes place which results in an undesired modulation of the current from this emitter. The frequencies of this undesired modulation are relatively low and if eliminated by the amplifier associated with the conductor 58 will result in the elimination of the undesired dark spot signal. Accordingly, the amplier including tubes I4, 'I6 and 'iB to which the video signals are applied from the electron multiplier by way of conductor 58, has a characteristic such that it amplies that portion of the video signal spectrum above approximately 200 kilocycles per second with the result that of coupling condenser |60. The distributed circuit of tube 82 is represented by the condenser C2. The tube 82 includes at least acathode, a control electrode and an anode and the cathode of this tube is connected to ground. The control electrode of tube 82 is connected to ground by way of grid resistances |62 and |55 while the junction of these resistances is connected to a source of negative potential by way of resistance |66. The potential drop across resistance |66, therefore, affords the necessary biasing for tube 82. The anode of tube 82 is connected to a source of positive potential by means of resistance |68 and, accordingly, amplified video signals are present at the anode of tube 82 as represented in general by curve |10 with signals representative of white extending in a negative direction and signals representing black extending in a positive direction.

It will be observed that the polarity of the video signals present in the anode circuit of tube 82 corresponds identically to the polarity of the signals present in the anode circuit of tube 18. The video signals present at the anode of tube 82 are applied to conductor |35 by way of coupling condenser |12 and their intensity may be altered by means of the potentiometer |14. Signals present on conductor |35 from the potentiometer |14 associated with the low frequency component amplifier including tubes il@ and 82 are, therefore, combined with the high frequency components of the video signal series by iniection into the anode circuit of tube 18 through condenser |36. There are, therefore, present at coupling condenser |15, frequencies representative of the complete video signal series and this complete .sienal series is represented bv curve |11 and is applied to the control eectrode of tube |38. Proper biasing for tube |38 is provided by means of the potential drop across resistance |18 and the control electrode of this tube is connectefi to ground through series resistances |18 and |80. The output from tube |38 mav then be applied to further stages of amplification, terminaticr with tube |92 where the amplified complete video signal series including the entire spectrum of frequencies mav be made available between output terminals |84 associated with tube |82.

Tubes 14, 16, 18 and |38 are preferably nentodes of a type similar to the type 6AK5 while tube 80 is preferably of the type 6AC7 and tube B2 may be of the type 6J6, which is a dual triode, with the tube operated by connecting the elements in parallel. It is not intended to imply that only these specifically mentioned tube types may be employed since it is obvious that different types of tubes may be `used so long as their characteristics are such as will afford and produce the desired results.

For practical purposes, it is desirable that the amplifiers above described for individually amplifying the high-frequency components and the low-frequency components of the entire video signal series possess the desired pass-band characteristics. To accomplish this purpose, the resistance-capacitance filters or networks which form a part of the amplifier circuits are very carefully chosen. The high-pass filter action of the amplifier to which signals are supplied from the multiplier by way of conductor 58, is obtained by decreasing the grid circuit time constant of tubes 16 and 18. The time constant associated with the grid circuit of tube 16 is determined by C3 and R3 while the time constant of the grid circuit of tube 18 is determined by C4 and R4. For the purpose of convenience, these two time constants Will be designated as T3 and T4 respectively. The low pass filter action of the amplifier to which signals are supplied from conductor 64 is obtained by normal load resistances-tube capacitances in the amplifier. The first load resistance is Ri (resistances M4 and |45 in series) and the associated condenser C| (the input tube capacitance of tube 80, together with some small distributed capacity between the wiring and ground) form a time constant circuit which will be designated as T|. Similarly, the load resistance R2 of tube 8E! together with the input capacitance C2 of tube 82 form a second time constant circuit T2. The input resistance RI and the time constant Tl are so chosen as to obtain a signal-to-nose ratio throughout the band of frequencies passed by this amplifier which is equivalent to or greater than that obtained from the electron multiplier in this particular range.

It has been found that to accomplish the desired results a particular relationship must exist between the time constants Ti, T2, T3 and Til. This relationship may be expressed as a ratio in which T|:T2=T3:T4. In other words, the product of TI and T4 should approximately equal the product of T2 and T3. In addition to this mathematical relationship, the values of the time constants must be so chosen that each amplifier will pass the desired bands of frequencies. For the particular values of circuit elements shown in Figure 1, the cut-off frequencies of the two amplifiers is chosen at approximately 200 kilocycles per second, as is shown in Figure 2 of the drawing. In this figure. a series of curves are shown representing the characteristics of the two amplifiers. The curve TL. for example, shows the response of the amplifier used to amplify the low frequency components of the video signal. series while the curve TH represents the respense of the amplifier associated with the electron multiplier of the Orthicon tube for amplifying the high frequency components. It will be observed that these two curves TL and TH cross at approximately 200 kilocycles per second and that the response of each amplifier is 1/ or 70.7% at this point. The characteristics of the high-frequency component amplifier must also be so chosen that an eective rapid decay of black level signals from the multiplier during retrace time will result in order to prevent the black level setter or clamp circuit, included in further stages of amplifier, from responding to the signal intensity of signals obtainable from the multiplier and the high frequency amplifier. It is desirable for proper black level setting that the clamp circuit respond to signals obtained from the target electrode over conductor 64 and the low-frequency component amplifier. To assure that a proper black level is established, it may in some instances, be necessary to delay the clamp action so that it occurs during the latter portion of the retrace interval and during the latter portion of the time that a blanking impulse is applied to the cathode and a control electrode of the electron gun in the Orthicon or camera tube. The blanking interval may be of the order of 6 microseconds to assure that the black level is determined by the signal intensity obtained from the target electrode and low frequency component 13 amplilerfat which time the amplitude vof 'the black level pulse passed by the high-frequency component amplifier has decayed to a negligible value due to the characteristics of that amplifier. Consideration of these particular problems, together with a satisfaction of the above mentioned relationship of the time constants, fairly definitely establish the permissible 'Values of circuit parameters that constitute the various time conetant circuits` As regards the matter of phase relationship, it will be observed that series and shunt peaking is employed in the amplifier associated with the multiplier of the Orthicon tube while no similar peaking is employed in the amplifier for amplifying the low-'frequency componen-ts of the video `signal series. The single high-gain stage including tube Bil of the lowpass amplifier, without the use of peaking coils, facilitates an exact match of complementary frequency and phase deviation in the two amplifiers.

Referring again to Figure 2, curve Ti represents the frequency response of the low-frequency component amplifier as a result of a time constant Tl while the curve T2 represents a response of the same amplifier as the result of the time constant T2. When both these time constant circuits are included in the amplifier, the resultant overall frequency response characteristic is such as that represented by the curve TL, Similarly, the curve T3 represents the response of the high-frequency component amplifier as a result of the time constant circuit T3 and the curve T4 represents the response of the circuit to the action of the time constant circuit Tl. When both these time constants are included in the amplier, the overall response may be such as that represented by the curve TH. The overall response of both amplifiers, when their outputs are combined. is represented .by the straight hor izontal line at the top of Figure 2.

Values of the various circuit parameters as indicated in Figure l of the drawing, are given by way of example only, since it is conceivable that some degree of selection is `possible even though certain rather stringent conditions must be met. Furthermore. the values of these elements, and especially the yal-ues of the elements that constitute the time constant circuits referred to, depend considerably upon the cut-olf lfrequency that is chosen. kilo-cycles per second were chosen, then naturally the values of the resistances and condensers constitutimT the time constant circuits would naturally be altered by a commensurate amount.

When two amplifiers are provided such as in- .dic-ated in Figure l, th'D output from the low frequency component amplifier may be combined with the output from the high frequency component amplifier by plate circuit injection as .represented in figure l and in the particular instance shown this is accomplished by coupling condenser E35 associated with the anode load circuit of tube le of the high frequency component am 'fier'. signal injection is show different types of signal it is conceivable that i jection or signal comb` may be exerc For example, it is @le to the sig. s from the low freer to the high iisqi ncy component amplifier by injection into the cathode circuit of tube 21S. Uncle this circumstance, however, and in View of the low im- .pedance associated with the cathode circuit of tube is, the circuit parameter associated with If a frequency other than 20!) 'r Although this specific type of ff `14 tube -82 `must also be altered in order that a proper impedance match may result.

The choice of the cut-off frequency of approx-imately 2G() kilocycles per second appears to be desirable although a higher or lower frequency kcut-off value could be employed. If a higher frequency were used an even more complete elimination of undesired spurious dark spot signals could .be accomplished although there is la, limit to a shifting of the cut-off frequency in this direction by reason of the signal-to-noise ratio at the grid of tube et as compared with the advantages obtained through the use of the electron multiplier portion of the Orthicon tube. If a lower frequency cut-off were employed, the undesired shading signal would not so completely be eliminated, but, on the other hand, a higher signalto-noise ratio would result in the low frequency 'component amplifier.

From the above, it may, therefore, be seen that undesired dark spot signals may be eliminated from an Orthicon tube employing an electron multiplier through the use of two parallel operating amplifiers which are supplied with video vsignals individually from the electron Vmultiplier portion of the Orthicon camera, tube and from the target electrode of the same tube. It may also be seen that signals from these two sources may be vpassed through appropriate amplifiers having desired band pass characteristics and that the amplifiers described herein permit the combining of the signals passed thereby in proper amplitude and phase relationship so that substantially no distortion occurs in the entire video signal series.

l' claim:

l. A pair of parallel operated band pass ampliiiers for amplifying television video signals comprising a rst amplier to which is yapplied a video signal series for amplifying the 10W frequency components of the video signals, said ampliner including at least one amplifying stage having an input load resistance-tube capacitance time constant Ti, Vsaid amplifying stage having output load resistance-tube capacitance time constant T2, a second amplifier to which is applied a substantially similar video signal series for amplifying the high frequency components of the video signals, said second ampliiier including at least two amplifying stages, each stage having elements constituting grid circuit time constants and Tal, the parameters of the two amplifiers being such that the product vof the time constants Ti yand is substantially equal to the product of the time constants T2 and T3, and in which time constants T3 and Til vare of the order of ten. times the time constants Tl and T2, respectively, and means for combining the outputs from the two amplifiers to produce a composite series of video signals.

2. A pair of parallel operated lamplifiers for amplifying television video signals comprising a first amplifier for amplifying the low frequency components of applied Video signal series, said amplifier including an amplifying stage having an input load resistance-tube capacitance circuit constituting time constant T! and an output load resistance-tube capacitance circuit constituting time constant T2, a second amplifier for amplifying the high frequency components of a substantially similar applied `video signal series, said second ampliuer including two amplifying stages, grid Acircuit v.elements associated with each amplifying stage constituting time constants T3 and Tt, the values of said load resistances and grid.

circuit elements being such that the product of the time constants Tl and T4 is substantially equal to the product of the time constants T2 and T3, and in which time constants T3 and T4 are of the order of ten times the time constants T l and T2, respectively, and means for combining the outputs from the two amplifiers to produce a complete composite series of video signals.

3. A television video signal amplifier including a pair of parallel operated band pass amplifiers for amplifying video signals comprising a first amplifier for amplifying the low frequency components of the applied video signal series, said amplifier including at least one amplifying stage, the load resistance for the applied video signals and the distributed capacity constituting a time constant Tl, an output load resistance for said amplifying stage, the load resistance and the tube capacitance of the amplifying stage constituting a second time constant T2, a second amplifier for amplifying the high frequency components of the applied video signal series, said second amplifier including at least two amplifying stages, each stage having an input coupling condenser and an associated grid resistance constituting time constants T3 and T4, the parameters of the time constant circuits being such that the ratio of the time constant TI to the time constant T2 is substantially equal to the ratio of the time constant T3 to the time constant T4, and in which the time constants Tl and T2 have a value of the order of one-tenth the time constants T3 and T4, respectively, and means for combining the video signals available at the outputs of the two amplifiers into a single composite series of video signals.

4. A television video signal dual channel amplifier including a pair of parallel operated amplifiers fcr amplifying substantially identical video signals comprising a first amplifier for amplifying the low frequency components of the applied video signals, said amplifier including an amplifying stage, a load resistance for the applied video signals, which, together with the distributed capacity constitutes a time constant Tl, an output load resistance for the amplifying stage, which, together with the tube capacitance constitutes a second time constant T2, a second amplifier for amplifying the high frequency components of the applied video signals, said second amplifier including two amplifying stages, each stage having an input coupling condenser and an associated grid resistance constituting time constants T3 and T4, the values of the resistances and capacitances of the two amplifiers being so chosen that the ratio of the time constant Tl to time constant T2 is substantially equal to the ratio of the time constant T3 to the time constant T4, and in which the time constants Tl and T2 have a value of the order of one-tenth the time constants T3 and T4, respectively, whereby, when the video signals available at the output of the two amplifiers `are combined into a single composite series of video signals, the composite signals will contain substantially no phase distortion.

5. A television video signal amplifier including a channel for amplifying the low frequency components of an applied video signal series and a parallel operated channel for amplifying the high frequency components of a substantially similar applied video signal series, said low frequency component amplifier including at least one amplifying stage having an input load resistancetube capacitance time constant circuit TI to which the video signals are applied and an output load resistance-tube capacitance time constant circuit T2 to which the signals passed by said amplifying stage are applied, said high frequency component amplifier including at least two amplifying stages, each of said amplifying stages being preceded by grid time constant circuits T3 and Tli, each time constant circuit comprising a series coupling condenser and a shunt grid resistance, the values of the elements constituting the various time constant circuits being s0 chosen that the product 0f the time constant circuits Tl and T4 is substantially equal to the product of the time constant circuits T2 and T3, and in which the time constants T3 and T4 are approximately ten times as large as time constants TI and T2, respectively, and means for combining the amplified video signals available from both of the amplifying channels to produce a composite series of video signals which is substantially free of phase distortion.

6. A television video signal amplifier including a pair of parallel operated amplifying channels, one channel for amplifying the low frequency components of a video signal series and the other channel for amplifying the high frequency components of a substantially similar video signal series, said one channel including an amplifying stage having an input load resistance-tube capacitance time constant circuit Tl to which the video signals are applied and an output load resistance-tube capacitance time constant circuit T2 to which the video signals passed by said amplifying stage are applied, said other channel including two amplifying stages, each amplifying stage being preceded by a series coupling condenser and a shunt grid resistance constituting time constant circuits T3 and T4, the values of the elements constituting the various time constant circuits being so chosen that the product of the time constant circuits TI and T4 is substantially equal to the product of the time constant circuits T2 and T3, and in which the time constants T3 and Ti are approximately ten times as large as time constants T! and T2, respectively, and means for combining the amplified video signals available from both of the amplifying channels to produce a composite series of video signals which is substantially free of phase distortion.

'7. A television transmitting system wherein an Orthicon image pickup tube having a target electrode and an electron multiplier is provided and wherein television video signals are derived from both the target electrode and the electron multiplier, comprising a low frequency component amplifier' to amplify the low frequency components of the television video signals available from the 'target electrode including at least one amplifying stage, the load resistance-tube capacitance circuit for the video signals available from the target electrode constituting a time constant circuit Tl and the load resistance-tube capacitance circuit to which the amplified signals are applied constituting a time constant circuit T2, a high frequency component amplifier to amplify the high frequency components of the television video signals available from the electron multiplier including at least two stages, each stage including input grid circuit time constant circuits T3 and Tf:- comprising series capacitances and shunt resistances, the values of the elements constituting the various time constant circuits being so chosen that the ratio of the time constant Tl to the time constant T2 is substantially equal to the ratio of the time constant T3 to the time constant T4, and in which the time constants Tl and T2 have a. value approximately one-tenth as large as the time constants T3 and T4, respectively, and means for combining the amplified video signals available from the outputs of the two amplier channels whereby a single composite series of video signals will be produced.

8. A television transmitting system wherein an Orthicon image pickup tube is used to provide video signals and wherein the tube has target electrode and an electron multiplier with provisions whereby television video signals may be derived from both the target electrode and the electron multiplier, comprising two parallel operated amplifying channels having diiferent band pass characteristics, one of which is eiective to amplfy the low frequency components of the television video signals available from the target electrode and the other of which is effective to amplify the high frequency components of the television video signals available from the electron multiplier, said low frequency component amplier including one amplifying stage, a load resistance-tube capacitance circuit constituting a time constant circuit Ti to which the video signals available from the target electrode are applied, a load resistance-tube capacitance time constant circuit T2 to which the signals ampliiied by the stage are applied, said high frequency component amplifier including at least two stages, each stage including series capacitances and shunt resistances constituting input grid circuit time constant circuits T3 and T4, the values of the elements constituting the various time constant circuits being so chosen that the ratio of the time constant Tl to the time consant T2 is substantially equal to the ratio of the time constant T3 to the time constant T4, and in which the time constants Tl and T2 have a value approximately one-tenth as large as the time constants T3 and T4, respectively, and means for combining the amplified video signals available from the outputs of the two amplifier channels whereby a single composite series of video signals will be produced.

9. A pair of parallel operated band pass amplifiers for amplifying television video signals comprising a, first amplifier to which is applied a video signal series for amplifying the frequency components of the video signal up to approximately 200 kilocycles per second, said amplifier including at least one amplifying stage having an input load resistance-tube capacitance time constant T and an output load resistance-tube capacitance time constant T2, a second amplier to which is applied a substantially similar video signal series for amplifying the high frequency components of the video signals above approximately 200 kilocycles per second, said second amplier including at least two amplifying stages, each stage having elements constituting grid circuit time constants T3 and T4, the parameters of the two amplifiers being such that the product of the time constants TI and T4 is substantially equal to the product of the time constants T2 and T3, and in which time constants T3 and T4 are of the order of ten times the time constants Tl and T2, respectively, and means for combining the outputs from the two amplifiers to produce a composite series of video signals, whereby the desired band pass characteristics are present and whereby a substantially distortion free composite series of video signals will result.

OTTO H. SCHADE.

REFERENCES CITED rThe following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,954,969 Wallace Apr. 17, 1934 2,134,851 Blumlein Nov. l, 1938 2,171,536 Bingley Sept. 5, 1939 2,177,366 Iams Oct. 24, 1939 2,182,578 Blumlein Dec. 5, 1939 2,192,959 Ballard Mar. 12, 1940 2,255,642 Artzt Sept. 9, 1941 FOREIGN PATENTS Number Country Date 454,383 Great Britain Sept. 29, 1936 490,391 Great Britain Aug. 15, 1938 

